em-ftp-client is a simple EventMachine based FTP Client supporting operations on stream data. The primary interface is EventMachine::FtpClient::Session. A standard usage would look like:

require 'eventmachine'
require 'em-ftp-client'

EM.run do
  EM::FtpClient::Session.new("0.0.0.0", 
    :username => "test", 
    :password => "1234") do |ftp|

    ftp.list do |l1|
      puts l1
      puts
      ftp.cwd("files") do
        ftp.list do |l2|
          puts l2
          puts
          ftp.put("one.txt") do
            ftp.stream {|d| puts "STREAMING: #{d.inspect}" }
            ftp.get("two.txt") do |t1|
              puts "COMPLETED"
              puts t1
              puts
              EM.stop
            end
          end
        end
      end
    end
  end
end

This library also includes the class SyncSession, which uses fibers and works with em-synchrony.